Following the 1808 reforms of the Prussian Army the line infantry regiments receive two musketeer battalions and a fusilier battalion.

I have conflicting information on the Reserve infantry regiments. Most formations appear to be three battalions strong, some are enhance to four. Did these include a fusilier battalion in all case? Or where they predominately just musketeer battalions? How many included a fusilier battalion?

Just painted Von Bulow 1813 Corps. All reserve units had fusiliers. All other things being the equal(and these uniforms never seem to be) the only difference was black belts for the fus, white for the mus. But not every battalion within a regiment had the same uniform, style or color. They had a mix of Prussian line or landwehr or British line or light or rifle (in both blue and green) or Portuguese or French.

IIRC there were 3 reserve Aux battalions( some with 4?) for the strammin regts and would be musketeer as a start. The transition was to have 1 of the 3 become functional as a fusilier btn and occurred during the 1813 campaign. The transition to have blacken strapping did not always work out, as indicated by the description of RES units 1813-1814 and even by 1815 per a fine post resource mentioned by PoE's posting and other great resources.A good question then would be when did these designated battalions fully functioned as fusiler and in a two rank formation ( as fusilier were to be 1800?-1816?)?

According to the 1812 regulations, all battalions would form in 3 ranks, including Füsiliere. When the 3rd rank was deployed as "Tirailleurs", the companies remained in 2 ranks. All battalions were trained to use the 3rd rank as skirmishers.